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Most advanced, high-power engine concepts that I came across aren’t suited for atmospheric launch, so I got curious about what kinds of engines could launch a very heavy craft from the surface of a planet like Earth or Mars. The best fit for this that I could find were Nuclear Thermal Rockets (https://en.wikipedia.org/wiki/Nuclear_thermal_rocket), mostly the solid core designs and the closed cycle gas core designs such as the Nuclear Lightbulb (https://en.wikipedia.org/wiki/Nuclear_lightbulb). Since the ideal use is in an atmosphere, an aerospike might help, and I managed to find at least one paper detailing such an engine design, using a solid core NTR with a toroidal chamber aerospike (https://arc.aiaa.org/doi/10.2514/6.2020-3841).

So from what I gather, it isn’t entirely crazy to go for a thermal rocket design when launching from an atmosphere. That said, delivering the heat directly to the propellant instead of conducting from a solid medium could be more efficient, since the melting of parts severely limits the maximum operating temperature, trapping the performance of solid core NTRs just around double that of chemical rockets. The Nuclear Lightbulb seeks to have a higher performance than solid core ones, with the Wikipedia stating that the core can reach 22000°C, but it delivers its energy to the propellant through intense UV, which avoids the melting problem as the container is transparent to it, but so is hydrogen, requiring it to be seeded with tungsten particles. This isn’t exactly efficient, which makes it just a bit better than solid core designs.

If the propellant could be heated directly, even to half of that temperature, the performance of the engine would far exceed that of a Nuclear Lightbulb. Avoiding the seeding of hydrogen would also make the propellant more convenient and more easily obtainable.

I thought of fusion, but all research that I could do only lead to designs suited for use in orbit (or at the very least, the use for atmospheric launch was never mentioned). I also remember reading in some comment or somewhere in this site that the propellant flow in the chamber could disturb the plasma confinement, but I couldn’t find it again, so I don’t have a link. I could be remembering wrong. If fusion IS valid for launching from Earth, though, I would love to understand the way the chamber would work, and the limitations involved (such as minimum weight and size for the engine, energy consumed, and so on).

I also looked into microwave thermal rockets: (https://www.researchgate.net/publication/30762215_Feasibility_and_Performance_of_the_Microwave_Thermal_Rocket_Launcher) (https://authors.library.caltech.edu/3304/1/PARaipcp04b.pdf) But the ones I could find were not heating the propellant directly, conducting it through a solid part instead, which brings back the melting-point limitations. I assumed it would be possible to deliver the microwaves inside a chamber, directly to the propellant, but I was having a hard time finding relevant results for this specific launch-capable context. I’m not even sure if that would be more efficient than through conduction, again, details would be appreciated.

Another thought was high-power lasers, using the best possible wavelength for absorption by the gas. I’m also pretty clueless on this front, and researching didn’t help me due to all the orbital pulsed designs. I’d love to know what that would look like (how the laser would enter the chamber, how to use it to heat the gas, and so on). I also thought of trying to take the Nuclear Lightbulb but replacing the fissile material with this “optimal absorption wavelength” idea, just having a very high-energy lightbulb. This was just an attempt to modify the concept as little as possible, but the details also elude me.

In short, my question is how do these methods compare and how could they be implemented, as well as whether there are better alternatives I’m missing. Ways to directly heat a simple gas propellant to very high temperatures.

Let’s assume that any parts in direct contact with the propellant (such as the chamber walls or the nozzle spike) can either have arbitrary thermal insulation, or enough active cooling to keep them from melting.

Let’s also assume the temperature delivered to the propellant to be between 10000 and 15000 K, with a mass flow rate of 500 kg/s.

For methods relying on electricity, we can assume a power source delivering between 1 and 100 gigawatts, but in a bottomless-well style (such as in the beamed propulsion ideas), but this can extend to other designs, just assume that the craft can use unlimited electric power.

Specifications on how efficient or wasteful (mass-wise) such alternative heat sources would be when compared to the heavy weight of fissile materials would be much appreciated!

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    $\begingroup$ Ground-takeoff NTR SSTOs have some serious feasibility problems due to the combination of a heavy and usually low-thrust engine, and the very low-density hydrogen propellant, plus the extreme cost of nuclear R&D in the modern era. Most nuclear lightbulb designs seem to be aimed at even lower thrust-weight ratios and are low-TRL. For a somewhat cynical look at things, see selenianboondocks.com/2010/06/ssto-ntr-bad $\endgroup$
    – ikrase
    Oct 1, 2022 at 23:30
  • $\begingroup$ @ikrase Yeah, I’m asking this exactly because the heavy weight of nuclear bothered me, as well as the heating methods limiting higher exhaust velocities. I also didn’t mention since the question was so big already, but in LH2 being too fluffy, I did some math (which could be very wrong) and apparently liquid methane or even ethanol are better. You lose a bit of efficiency but can pack a lot more fuel, which actually gives longer burn times. Everything should dissociate into atoms at such temperatures, but I know methane does, at least. $\endgroup$ Oct 2, 2022 at 1:52
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    $\begingroup$ Anything other than hydrogen loses an awful lot of efficiency, really. Methane is problematic because carbon is a solid and you get soot. $\endgroup$
    – ikrase
    Oct 2, 2022 at 3:05
  • $\begingroup$ I think hydrogen is already the best way to convert UV (and gamma/x- rays) to longer waves already. And the energy out of the core must be UV or shorter because longer waves can't move energy out fast enough unless with a larger emission area. $\endgroup$ Oct 2, 2022 at 3:46
  • $\begingroup$ What's up with the four unexplained close votes? $\endgroup$
    – Ingolifs
    Oct 4, 2022 at 3:21

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